We implemented a graphical user interface in an X-window/UNIX environment to compute and display the incoherent average of Fourier amplitudes from electron images of single particles embedded in ice and the simulated contrast transfer function with or without envelope functions. This interface provides an easy and efficient method for the determination of the defocus value of the contrast transfer function and of the extent of the Fourier amplitude fall-off. This computational procedure is crucial for pre-screening image data and performing image correction in high resolution three-dimensional reconstruction of single particles. This software can be obtained on the WWW at http://condor.bcm.tmc.edu/3DEM/ftp.html. PTOOL, a suite of tools packaged in an X window system based graphical user interface has been developed to allow consistent and accurate selection of single particles from low contrast spot-scan and flood beam electron microscopy images. These user friendly and computationally efficient tools run on UNIX based workstations. Particles are selected using an automatic reference based selection method which incorporates extensive image preprocessing. An image division and subsequent image sewing mechanism allows for efficient particle selection independent of micrograph size. A particle viewing tool is provided which displays galleries of images and has options for performing image processing operations such as Fourier filtering, histogram equalization, and image masking. In addition, a tool is included to perform semi-automatic focal pair particle matching to identify the same particles on two micrographs. This software package may be obtained on the WWW at http://condor.bcm.tmc.edu/3DEM/People/pam/ pam.html.